This invention relates to a process for the production of methacrylic or acrylic acid by oxidizing methacrolein or acrolein.
More particularly, this invention relates to a process for the production of methacrylic or acrylic acid by oxidizing methacrolein or acrolein with molecular oxygen in the presence of water vapor characterized in that the reaction system is provided with a catalyst which contains palladium, phosphorus and oxygen as essential elements and antimony as an optional element and that a phosphoric acid or a phosphorus compound capable of forming a phosphoric acid through a chemical change during the reaction is concurrently supplied to the reaction system.
For the synthesis of methacrylic acid by oxidizing methacrolein in vapor phase a number of catalysts have hitherto been proposed.
Almost all of these catalysts, however, have low catalytic activities. Further, if the reaction is carried out at elevated temperatures in order to increase the total rate of reaction, these catalysts are degraded or decomposed at such elevated temperatures to yield large amounts of undesirable by-products such as carbon monoxide, carbon dioxide, etc. so that the per-pass yield of methacrylic acid is very low.
The catalyst as disclosed in the Japanese Patent Laid-open Publication Nos. 67216/1973 and 61416/1973, which are improved in their catalytic activity and selectivity, comprise phosphomolybdic acid or salts thereof as a main ingredient.
The phosphomolybdic catalyst has serious disadvantages that it is short in its service life and thermally unstable so that the catalytic activity starts to rapidly decrease at a reaction or calcination temperature of more than 450.degree. C. Once the catalyst has been deteriorated, it can be regenerated no more by a simple treatment, for example, by calcining it again. These facts show that the phosphomolybdic catalyst is not always available for commercial use.
Further, when the reaction is to be carried out at high space velocities, the service life of the phosphomolybdic catalyst will considerably be reduced.
From a commercial aspect, it is desired to develop a catalyst which has acceptable activity and selectivity at low temperatures as well as a longer service life. Particularly desirable is a catalyst which can maintain its activity over a long period of time even when the reaction is carried out at high speed velocities.